Source contributions of surface ozone in China using an adjoint sensitivity analysis

Air pollution has become an adverse environmental problem in China, resulting in serious public health impacts. This study advanced and applied the CMAQ adjoint model to quantitatively assess the source-receptor relationships between surface ozone (O₃) changes over different receptor regions and precursor emissions across all locations in China. Five receptor regions were defined based on the administrative division, including northern China (NC), southern China (SC), Pearl River Delta region (PRD), Yangtz River Delta region (YRD), and Beijing-Tianjin-Hebei region (BTH). Our results identified the different influential pathways of atmospheric processes and emissions to O₃ pollution. We found that the atmospheric processes such as horizontal and vertical advection could offset the O₃ removal through chemical reactions in VOC-limited areas inside the receptor regions. In addition, O₃ pollution can be induced by transport of O₃ directly or its precursors. Our results of relative source contributions to O₃ show that transboundary O₃ pollution was significant in SC, NC and YRD, while the O₃ pollution in PRD and BTH were more contributed by local sources. Anhui, Hubei and Jiangsu provinces were the three largest source areas of NO_x and VOC emissions to O₃ in SC (>52%) and YRD (>69%). NO_x and VOC emissions from Tianjin and Beijing were the largest contributors to O₃ in NC (>34%) and BTH (>51%). PRD was the dominant source areas of NO_x (>89%) and VOC emissions (~98%) to its own regional O₃.